Input system for electrical amplifiers



y 23, 1933- H. A. SNOW INPUT SYSTEM FOR ELECTRICAL AMPLIFIERS OriginalFiled July 1929 kw. 0 W 3 Wan/e L's/797% 2 1 m 62. dm-

Patented May 23, 1933 UNE'EEE) STATES FATENT FFICE HAROLD A. SNOW, OFMOUNTAIN LAKES, NEW JERSEY, ASSIGNOR, BY MESNE ASSIGN- IVIENTS, TO 35310COEPOEATIGN OF AMERICA, F NEW YORK, N. Y., A. CORPORA- TION O1 DELAWAREINPUT SYSTEM FOR ELECTRICAL AMPLIFIERS Application filed July 5, 1929,Serial No. 328,130. "Renewed October 24, 1931.

This invention relates to input systems for electrical amplifiers andparticularly to input systems for use with radio frequency amplifiers ofthe type commonly employed in radio receiving sets.

Input systems have been proposed in which the signal energy wasimpressed across a fixed condenser of relatively large capacity thatconstituted a series element in a condenser-tuned resonant circuit. Whenthe fixed condenser has a value substantially greater than the maximumvalue of the variable condenser, the input system tends to equalize thevoltage step-up or gain throughout the band to which the system may betuned. Unfortunately, however, the fixed condenser can not be given suchrelatively large value as might be indicated from a consideration ofonly the gainwavelength characteristic, since the fixed condenser shuntsthe signal energy to ground and thereby reduces the voltage availableacross the fixed condenser.

An object of the invention is to provide an eflicient input systemincluding a resonant circuit operating into the first radio frequencyamplifier, the input system being of such construction and arrangementthat the tuning characteristics of the resonant circuit aresubstantially independent of the antenna impedance.

A further object of the invention is to provide an input system of thetype in which the signal energy is impressed across a fixed condenserconstituting a series ele ment in a condenser-tuned resonant circuit,and in which the magnitude of the fixed condenser may be such as toproduce a de sired gain-wwrelength characteristic without, however,acting as a by-pass for signal energy.

These and other objects of the invention will be apparent from thefollowing specification when taken with the accompanying drawing inwhich:

Fig. lis a fragmentary circuit diagram illustrating one embodiment ofthe inven- 'tiOn;

Fig. 2 is a circuit diagram of a modified form of input system; and

Fig. 3 comprises curves representing the performance of the input systemof Fig. 2.

In the drawing, the reference character C indicates the fixed condenserwhich forms a series element of the resonant circuit LC. As indicated inthe drawing, the inductance L may be adjustable for the purpose ofmaking the tuning characteristics of the first stage identical withthose of the succeeding stages, but the tuning of the circuit over aband of frequencies is effected by means of the variable condenser C.

In accordance with the prior practice, the antenna lead was connecteddirectly to the junction of inductance L and fixed condenser C but whenthe capacity of C was of the order of .001 Inf, the bypassing effect ofthe condenser was decided ly noticeable. So far as the gain-wavelengthcharacteristic of the system was concerned, better operation would beeffected if the fixed condenser were given an even greater value, butthe by-pass effect precluded any substantial increase in the magnitudeof the condenser.

In accordance with the present invention, the signal energy of theantenna or collector system A is impressed across the fixed condenser Cby means of a stepdown transformer L L the turn ratio of the transformerbeing, for example, of the order of to 1.

Due to the transformer action, the apparent impedance of the fixedcondenser C is approximately 100 times the actual impedance, andtherefore the by-passing of signal energy must be substantiallyprevented even though the actual value of O is materially increased. Forthe broadcast band, the value of C may be .05 mf with a value of .0005for the maximum capacity of the tuning condenser C.

Due to the high capacity of the coupling condenser C the tuningcharacteristics of the resonant circuit may be made substan tiallyidentical with those of the succeeding stages and the condenser C of thefirst amplifier may be mechanically connected to the condenser orcondensers G which tune the input circuits of succeeding stages. Forexample, when C is of the order of .05 microfarads and the inductance Lhas the same physical and electrical values as the secondaries of theinter-stage transroriners T, any error introduced by varying the tan ingcapacities C and C, at the same rate w ll not exceed the usual tolerancepermitted in sensitive receivers.

As shown in Fig. l, the interstage coupling comprises a transformer Tand the tuning condenser Q, shunted across the secondary of thetransformer. It will be apparent that other iuned interstage couplingsmay be employed.

As shown in Fig. :2, the step-down transformer across which the signalenergy is impressed may take the form of an auto transformer comprisingthe coils L and I Although the invention. is not limited to anyparticular values for the circuit elements, the following data is givenby way e'li example:

L ==1O0 turns of No. 38 enamel wire close wound on 4; inch diametertube;

11 turns of No. 30 D. C. C. wire space wound on top of L As statedabove, the operation of the input s stem is characterized by a flattenng of the step-up characteristic and this action is particularlydesirable since the usual types of tuned radio freipiency amplifiershave a decidedly higher gain at the shorter wave-lengths. .Vhile it ispossible to construct amplifier stages having a relatively flatgain-wavelength charactcritic, the provision of an input system whichreduces the step-up at the shorter waves gives greater freedom in thedesign of the amplifier.

The curves of Fig. 3 are plotted from data obtained with an input systemof the type shown in Fig. i, the transformer being constructed inaccordance with the data given above, and the condenser C having a valueof .05 mt. The three separate curves A, B and C represent the step-upcharacteristics when the input system was associated with antennastructures in which the antenna capacity had tie respective values ofapproximately 100, 200 and 500 innit. It will e noted that, as comp redw: h the step-up ch racterisiics of the usual input systems, the presentinvention provides a substantially uniform step-up throughout the bandof frequencies to which the resonant circuit may be tuned.

By an appropriate choice of the values for the elements of the inputsystem, the step-up characteristic may be given a definite andpredetermined form. For a particular value of the antenna capacity (1,,the leakage inductance of {he transformer and the value of the condenserC ,may be such that the leakage inductance and U, are resonant at thelower end of the wavelength band, (note curve C of Fig. 3), and

the leakage inductance and C are resonant at the upper end of the band(curve A, Fig.

Since the large value of condenser Cm renders the tuned circuitsubstantially independent of the antenna characteristics, the alignmentof the ganged tuning condensers C, C will not be disturbed if a volumecontrol resistance R is shunted across the condenser C Although theinvention is particularly useful when employed as a coupling between anantenna and the first stage of a cascaded radio frequency amplifier, thefact that the tuning characteristics of the resonant circuit may berendered substantially independent of the impedances out of which theamplifier works indicates other useful applications of the invention.The novel form of input system may, for example, provide a couplingbetween a section of a carrier wave transmission line and a repeater, orit may be employed as a coupling between the plate circuit of one tubeand the input circuit of the following tube.

I claim:

1. A vacuum tube input system of the type including a series-resonantcircuit comprising av fixed condenser, a variable condenser and aninductance, coupling means for impressing signal energy across saidfixed condenser, and means for applying the voltage across said variablecondenser to the input terminals of a vacuum tube, characterized by thefact that said coupling means includes a transformer effective toincrease the apparent impedence of said fixed condenser.

2. In an input system for a radio frequency amplifier, a collectorsystem, a tuned circuit comprising in series a fixed condenser, aninductance and a variable condenser, means for grounding the junction ofsaid fixed and variable condensers, means for applying the voltageacross said Variable condenser to the input terminals of a radiofrequency amplifier, and coupling means for impresing the signal eneregyof said collector system across said fixed condenser, said couplingmeans con'iprising a transformer for increasing the apparent impedanceof said fixed condenser.

3. In an audion innput system of the type in which signal energy to beamplified is impressed across av fixed condenser forming a serieselement of a condenser-tuned circuit, a connection from the lowpotential side of said fixed condenser to ground, and a step-downtransformer having the secondary thereof connected across saidcondenser, the primary having one terminal connected to ground andconstituting the impedance across which received signal energy isimpressed.

4. The invention as set forth in claim 2, wherein the leakage inductanceof'said transformer and the said fixed capacity are resonant at theupper end of the band of wavelengths to which said circuit may be tuned.

5. The invention as set forth in'claim 2, wherein the leakage inductanceof said transformer and the capacity of said collector system are sorelated. as to be resonant at the lower end of the band of wavelengthsto which said circuit may be tuned.

6. The invention as set forth in claim 2, wherein the leakage inductanceof said transformer and the said fixed capacity are resonant at theupper end of the band of wavelengths to which said circuit may be tunedand the leakage inductance and capacity of said collector system areresonant at the lower end of said band.

7. In an input system for a vacuum tube, the combination with a tunedcircuit com prising in series a fixed condenser, an inductance and aVariable condenser; means grounding the junction of said fixed andvariable condensers and means for applying the voltage developed acrosssaid variable condenser to the input terminals of a vacuum tube; ofcoupling means for impressing upon said fixed condenser the signalenergy to be amplified, said coupling means comprising a transformer forincreasing the apparent impedance of said fixed condenser.

8. The invention as set forth in claim 7, wherein the leakage inductanceof said transformer and the said fixed capacity are resonant at theupper end of the band of wavelengths to which said circuit may be tuned.

9. The invention as set forth in claim '7, in combination with atransmission channel connected across the primary of said transformer,and wherein the leakage inductance of said transformer and the capacityof the transmission channel are so related as to be resonant at thelower end of the band of wavelengths to which said circuit may be tuned.

10. The invention as set forth in claim 7, in combination with atransmission channel connected across the primary of said transformer,and wherein the leakage inductance of said transformer and the capacityof the transmission channel are so related as to be resonant at thelower end of the band of wavelengths to which said circuit may be tuned,and the said leakage inductance and said fixed capacity are resonant atthe upper end of said band.

11. In combination, a source of modulated carrier energy, a multistagecarrier frequency amplifier, each stage including an electron dischargetube provided with input and output electrodes, a resonant circuitconnected between the input electrodes of the first of said amplifiertubes, said circuit comprising in series a fixed condenser andinductance, and a variable condenser, a resonant circuit connectedbetween the input electrodes of a succeeding tube and including avariable condenser, means for simultaneously varying said condensers,and means for transmitting energy from said source across said fixedcondenser comprising a transformer arranged to increase the apparentimpedance of the fixed condenser.

12. In combination, a source of modulated carrier energy, a multi-stagecarrier frequency amplifier, each stage including an electron dischargetube provided with input and output electrodes, a resonant circuitconnected between the input electrodes of the first of said amplifiertubes, said circuit comprising in series a fixed condenser, a variableinductance, and a variable condenser, a resonant circuit connectedbetween the input electrodes of a succeeding tube and including avariable condenser, means for simultaneously varying said condensers,and means for transmitting energy from said source across said fixedcondenser comprising a transformer arranged to increase the apparentimpedance of the fixed condenser.

13. In combination, a source of modulated carrier energy, a multi-stagecarrier frequency amplifier, each stage including an electron dischargetube provided with input and output electrodes, a resonant circuit connected between the input electrodes of the firstof said amplifier tubes,said circuit comprising in series a fixed condenser and inductance, anda variable condenser, a resonant circuit connected between the inputelectrodes of a succeeding tube and including; a variable condenser,means for simultaneously and similarly varying said condensers, andmeans for transmitting energy from said source across said fixedcondenser comprising a transformer arranged to increase the apparentimpedance of the fixed condenser.

14. In combination, a source of modulated carrier energy, a multi-stagecarrier frequency amplifier, each stage including an electron dischargetube provided with input and output electrodes, a resonant circuitconnected between the input electrodes of the firstof said amplifiertubes, said circuit comprising in series a fixed condenser andinductance, and a variable condenser, a resonant circuit connectedbetween the input electrodes of a succeeding tube and includlng avariable condenser, means for simultaneously varying said condensers,and means for transmitting energy from said source across said fixedcondenser comprising a step-down transformer arranged to increase theapparent impedance of the fixed condenser.

15. In combination, a source of modulatprovided with input and outputelectrodes,

a resonant circuit connected between the input electrodes of the saidamplifier tube, said circuit comprising in series a fixed condenser andinductance, and a variable condenser, and means for transmitting energyfrom said source across said fixed condenser comprising a transformerarranged to increase the apparent impedance of the fixed condenser, saidtransformer consisting of a pair of coils connected in series.

16. In combination, a source of modulated carrier energy, a carrierfrequency amplifier including an electron discharge tube provided withinput and output electrodes, a resonant circuit connected between theinput electrodes of said amplifier tube, said circuit comprising inseries a fixed condenser and inductance, and a variable candenser, andmeans for transmitting energy from said source across said fixedcondenser comprising a transformer having a turn ratio of the order ofto 1 arranged to increase the apparent impedance of the fixed condenser,and a gain control impedance connected across said fixed condenser.

17. In combination, a source of modulated carrier energy, a carrierfrequency amplifier including an electron discharge tube provided withinput and output electrodes, a resonant circuit connected between theinput electrodes of said amplifier tube, said circuit comprising inseries a fixed condenser having a value of the order of 0.05 uncrofaradand inductance, and a variable condenser, and means for transmittingenergy from said source across said fixed condenser comprising atransformer arranged to increase the apparent impedance of the fixedcondenser.

18. In combination, a grounded antenna circuit, a stage of radiofrequency amplification including a tube provided with input and outputcircuits, said tube input circuit comprising an inductance coil shuntedby a variable tuning condenser, means for coupling said antenna circuitand said tube input circuit in such a manner as to render the tuning ofsaid tube input circuit substan tially independent of the antennacharacteristics, and a variable impedance having one terminal thereofconnected to one side of said coil and its other terminal connected tothe grounded side of the said antenna circuit.

19. In combination, a grounded antenna circuit, a stage of radiofrequency amplification including a. tube provided with input and outputcircuits, said tube input circuit comprising an inductance coil shuntedby a variable tuning condenser, means for coupling said antenna circuitand said tube input circuit in such a manner as to render the tuning ofsaid tube input circuit substantially independent of the antennacharacteristics, and a variable impedance having one terminal thereofconnected to one side of said coil and its other terminal connected tothe grounded side of the said antenna circuit, said impedance beingconnected across said variable condenser.

20. In combination, a grounded antenna circuit, a stage of radiofrequency amplification including a tube provided with input and outputcircuits, said tube input circuit comprising an inductance coil shuntedby a variable tuning condenser, means for coupling said antenna circuitand said tube input circuit in such a manner as to render the tuning ofsaid tube input circuit substantially independent of the antennacharacteristics, and a variable impedance having one terminal thereofconnected to one side of said coil and its other terminal connected tothe grounded side of the said antenna circuit, and a second tuned radiofrequency amplifier stage, coupled to the output circuit of said firststage, including a variable tuning condenser in its input circuit, andmeans for simultaneously varying the variable condensers of both stages.

21. In combination, a grounded antenna circuit, a stage of radiofrequency amplification including a tube provided with input and outputcircuits, said tube input circuit comprising an inductance coil shuntedby a variable tuning condenser, means for coupling said antenna circuitand said tube input circuit in such a manner as to render the tuning ofsaid tube input circuit substantially independent of the antennacharacteristics, and a variable impedance having one terminal thereofconnected to one side of said coil and its other terminal connected tothe grounded side of the said antenna circuit, said antenna circuit andcoupling means being desi ned to be non-resonant over asubstantialportion of the operating range of the tuning condenser.

22. In combination, a source of modulated radio frequency energy, anelectron discharge tube provided with input and output electrodes, aresonant network, comprising an inductive path and a shunt variablecapacitive path, connected between the input electrodes, a largecapacity connected across a portion of said inductive path, and meansfor inductively coupling said source to said portion.

23. In combination, a source of modulated radio frequency energy, anelectron discharge tube provided with input and output electrodes, aresonant network, comprising an inductive path and a shunt variablecapacitive path, connected between the input electrodes, a largecapacity connected across a portion of said inductive path, and meansfor inductively coupling said source to said

